1. Field of the Invention
The present invention relates to an optical disk that is used for information recording or reproduction and further to a method of magnetically separating tracks of the optical disk.
2. Related Background Art
It has been desired in the area of optical disks to achieve higher-density information recording. A method for realizing such high-density recording has been proposed in the form of a DWDD (domain wall displacement detection) type optical disk.
In the DWDD type optical disk, it is necessary to weaken magnetic coupling between adjacent recording/reproducing tracks. Therefore, when manufacturing the DWDD type optical disk, prior to recording of information signals, a magnetically separating process for magnetically separating adjacent recording/reproducing tracks is performed. The magnetically separating process is performed by methods such as those disclosed in JP 6(1994)-290496 A and JP10(1998)-340493 A.
A structure of the DWDD type optical disk and a method of magnetically separating process according to a conventional technique are described as examples with reference to FIG. 7. In an optical disk shown in FIG. 7, a first dielectric layer 702, a magnetic layer 703, a second dielectric layer 704, and a protective coating layer 705 are laminated in this order on a substrate 701. On a surface of the substrate 701 on a side of the thin film layers, grooves 706 are formed. A portion between two of the grooves 706 adjacent to each other in a radial direction is referred to as a land 707 that is used as a recording/reproducing track. The grooves 706 have a width of, for example, 0.2 xcexcm, and the lands 707 have a width of 1.4 xcexcm. The magnetic layer 703 includes at least three magnetic thin films that are used for reproduction employing a DWDD system. In order for the DWDD system to be performed, it is necessary to magnetically separate the lands 707 as the tracks used for recording/reproduction.
The following description is directed to a method of performing the magnetically separating process with respect to this optical disk. In the magnetically separating process, a laser beam 708 to be used for annealing is focused on the groove 706 by an objective lens 709 and allowed to scan along the grooves 706, so that magnetic coupling between the magnetic layers 703 on the grooves 706 is lost. As a result, in each of the lands 707, which is a region interposed between the grooves 706 that have been subjected to annealing, both sides of the land 707 is magnetically separated, thereby allowing a DWDD operation to be performed. The laser beam 708 used in this process has, for example, a laser power of 2 mW and a wavelength xcex of 780 nm. The objective lens 709 has a NA of 0.5, and a beam spot of about 800 nm in diameter is formed. The beam spot of the laser beam 708 travels at a speed of, for example, 2 m/second.
When the lands 707 interposed between the grooves 706 are used as the recording/reproducing tracks as in the foregoing description, at an innermost or outermost end of a recording/reproducing track region, the groove 706 is provided. That is, even the land 707 positioned at the innermost or outermost end is interposed between the grooves 706. Accordingly, both sides of each of all land tracks can be magnetically separated by allowing the laser beam 708 for annealing to scan over all the grooves 706.
When manufacturing a disk provided with grooves, generally, portions exposed to a laser beam during master disk cutting correspond to the grooves of the disk. In the master disk cutting, due to a limit in the degree to which a laser beam is converged, it is difficult to manufacture grooves of a width as narrow as 0.1 to 0.2 xcexcm. When a laser having a wavelength of 351 nm and an objective lens having a NA of 0.9 are used as in conventional cutting, an obtained groove width is inevitably not less than 0.3 xcexcm. However, as a region for magnetically separating tracks so as to enable the DWDD operation, it is sufficient to have a width of about not less than 0.02 xcexcm, which corresponds to a width of a domain wall.
In order for an excellent DWDD operation to be performed, a domain wall driving force is required to be greater than a domain wall displacement inhibiting force, and thus a recording/reproducing track is required to have a width of 0.3 to 0.6 xcexcm. Accordingly, when lands are used as the recording/reproducing tracks, a track pitch is inevitably not less than 0.6 xcexcm.
Alternatively, the grooves may be used as the recording/reproducing tracks, with the lands being subjected to the magnetically separating process. In this case, the lands can be reduced in width to 0.1 to 0.2 xcexcm while maintaining the width of the grooves of 0.3 to 0.5 xcexcm, which is advantageous from a processing standpoint.
When the grooves are used as the recording/reproducing tracks, annealing is performed by allowing a light beam to scan over the lands, so that groove tracks are magnetically separated. In this case, it is possible to allow the light beam to scan over the lands of a narrower width by operating tracking servo using 1st-order diffracted light diffracted from the lands. However, on an innermost track and an outermost track in a recording/reproducing track region, the following problem arises.
In a conventional technique in which the lands are used for recording, since the groove always is formed on an inner side of an innermost recording/reproducing track, annealing is performed by tracking on the groove, so that the magnetically separating process can be performed with respect to both sides of an innermost land track.
On the other hand, when the grooves are used for recording, there is difficulty in performing the magnetically separating process as follows. As for an innermost groove track, for example, on an outer side of the groove track, the land is formed; however, on an inner side of the groove track, a flat surface spreads, on which no land region of a narrower width is formed. Therefore, while the magnetically separating process can be performed with respect to the land on the outer side of the innermost groove track, the magnetically separating process hardly can be performed on the inner side of the groove track, because tracking servo cannot be operated on the same side. The same applies to an outermost groove track. While the magnetically separating process can be performed with respect to an inner side of the outermost groove track, it hardly can be performed with respect to an outer side of the groove track.
In order to solve the aforementioned problem, it is an object of the present invention to provide an optical disk that achieves high-density recording and allows an excellent DWDD (domain wall displacement detection) operation to be realized by performing a magnetically separating process with respect to both sides of each of all recording/reproducing tracks provided in a region extending from an innermost track to an outermost track, and a method of performing the magnetically separating process with respect to the optical disk.
An optical disk of the present invention has a basic configuration including a disk-shaped substrate with pits and grooves formed by emboss processing, and at least a first dielectric layer, a magnetic layer, and a second dielectric layer that are formed on the substrate. In the substrate, a predetermined region ranging in a radial direction is used as a data region for recording/reproducing data. The data region includes a plurality of recording/reproducing tracks ranging from an innermost track to an outermost track, and is divided into a plurality of segments in a tangential direction. Each segment includes a pit region and a groove region. The pit region is provided with at least a pair of wobble pits to be used for sample servo. The groove region is provided with the recording/reproducing track. The recording/reproducing tracks are composed of the grooves. The magnetic anisotropy of the magnetic layer positioned on each of lands between the respective recording/reproducing tracks is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the grooves, so that the recording/reproducing tracks are magnetically separated. The aforementioned is common to optical disks of the present invention as the basic configuration.
An optical disk of a first configuration according to the present invention has, in addition to the aforementioned basic configuration, a feature that at least one inner dummy track that is not used for data recording/reproduction is provided on the inner side of the innermost recording/reproducing track, and at least one outer dummy track that is not used for data recording/reproduction is provided on the outer side of the outermost recording/reproducing track.
According to this configuration, the following advantage can be achieved. That is, since the recording/reproducing tracks are composed of the grooves, a narrower track pitch can be realized while a sufficient width of the grooves is maintained. Further, since the inner dummy track is provided, the land of a narrower width also can be formed on the inner side of the innermost recording/reproducing track. Thereby, on the lands on both sides of the innermost recording/reproducing track, tracking servo of a light beam to be used for the magnetically separating process can be operated by using a conventional push-pull detection method. Thus, both sides of the innermost recording/reproducing track also can be magnetically separated. The same applies to the outermost recording/reproducing track.
For the realization of high-density recording, it is advantageous to form magnetically separating regions so as to have the narrowest possible width. For the formation of the magnetically separating regions having the narrowest possible width, it is effective to form a light beam spot to be used for the magnetically separating process so as to have the smallest possible diameter. Therefore, it is desirable to use a short wavelength laser such as a blue laser, a high NA objective lens, or the like. Particularly, when the high NA objective lens is used, because of a short working distance of the lens, it is advantageous to irradiate the light beam to be used for the magnetically separating process onto the land from a film surface side rather than via the substrate. This achieves an improvement in energy efficiency and allows high-speed processing.
An optical disk of a second configuration according to the present invention has, in addition to the aforementioned basic configuration, a feature that the magnetic anisotropy of the magnetic layers positioned on a land portion on the inner side of the innermost recording/reproducing track and a land portion on the outer side of the outermost recording/reproducing track, respectively, is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the recording/reproducing tracks. Thus, the magnetic separation is achieved on the inner side of the innermost track and the outer side of the outermost track.
An optical disk of a third configuration according to the present invention has, in addition to the aforementioned basic configuration, a feature that also in the pit region, the magnetic anisotropy of the magnetic layer positioned on an extension of the land is reduced to a level lower than that of the magnetic anisotropy of the magnetic layer positioned on an extension of the groove.
An optical disk of a fourth configuration according to the present invention has, in addition to the aforementioned basic configuration, a feature that the grooves are composed of a discontinuous series of grooves in the form of a spiral that is interrupted in the pit regions at an interval of not more than 7 xcexcm.
According to this configuration, the magnetically separating process can be realized efficiently using a simple device, in which groove tracks are magnetically separated over the entire data region by allowing a light beam to scan over the lands of a narrower width. That is, according to this configuration of the grooves in the spiral form, the magnetically separating process can be completed, for example, in the following manner. On the land on an inner side of the innermost track in the data region, tracking servo is operated using a light beam to be used for the magnetically separating process by the conventional push-pull detection method. The tracking servo can be operated so that a magnetically separating operation is continued and completed beyond the outermost track in the data region. These operations can be performed as a series of operations without performing switching, a track jump, or track seeking during the operations. Thus, in the data region, the magnetic separation can be achieved on both sides of each of all the tracks ranging from the innermost track to the outermost track.
Each of the lands that are subjected to the magnetically separating process may be formed intermittently. However, it is required that the interval at which the land is interrupted be not more than 7 xcexcm for the following reason. The maximum eccentricity of the disk or the maximum eccentricity of the disk with chucking errors or the like being considered is expected to be 70 xcexcm. If the interval at which the land is interrupted is not more than 7 xcexcm with respect to the above maximum eccentricity, less influence is exerted by each interruption of the land, thereby allowing a light beam to be lead smoothly back to a portion of the next succeeding land. According to this configuration, even in the case where the disk has the maximum eccentricity, an amount of off-tracking caused during each interruption of the land is 0.05 xcexcm at most. Thus, even when the lands are formed intermittently, the tracking functions reliably, thereby allowing the magnetically separating process to be completed by performing the series of operations in a continuous manner throughout the process.
In any of the optical disks of the aforementioned configurations, the following is preferable. The magnetic layer includes at least a domain wall displacement layer, a switching layer, and a recording layer that are formed in this order. The switching layer has a Curie point lower than those of the domain wall displacement layer and the recording layer. In a region having a temperature higher than the Curie point of the switching layer, a domain wall transferred to the domain wall displacement layer shifts toward a high-temperature portion.
Furthermore, in any of the optical disks of the aforementioned configurations, preferably, a track pitch is not more than 0.6 xcexcm.
A method of magnetically separating tracks of an optical disk according to the present invention is a method of magnetically separating the recording/reproducing tracks when any of the optical disks of the aforementioned configurations is provided. In the method, a light beam converged to a degree higher than that of a light beam to be used for recording/reproduction is allowed to scan over the lands that correspond to a portion between the recording/reproducing tracks, so that the magnetic anisotropy of the magnetic films positioned on the lands or both of the lands and the extensions of the lands is reduced to a level lower than that of the magnetic anisotropy of the magnetic films positioned on the grooves.
Using an optical disk subjected to the magnetically separating process as in the aforementioned configuration, a stable DWDD operation is realized. Further, when the optical disk has the configuration in which the magnetic anisotropy of the magnetic films positioned both on the lands and on the extensions of the lands is reduced to a level lower than that of the magnetic anisotropy of the magnetic films positioned on the grooves, the following effect can be achieved in addition to the effect of realizing the stable DWDD operation in each track. That is, a DWDD operation can be performed in flat portions in which the lands are interrupted. Thereby, when the DWDD operation does not work well in the groove tracks, it can be determined whether the inferiority is caused by a defect in a film configuration or a defect in the shapes of the lands and the grooves by comparing the DWDD operations performed in the groove tracks and the flat portions, respectively.